Monthly Archives: April 2015


The Scott 222C Restoration, Part 3

Welcome back! You can find Part 2 here and Part 1 here.

At this point I decided to load all the tubes up and run a voltage test, verifying the operation of the power supply and the circuit as a whole. Now, the original circuit requires two pairs of matched tubes (ideally a matched quad). I have a modified Eico 667 tube tester which has a set of tip jacks added to it that allow me to measure cathode current while the tube is under test (I’ll give that its own post in the future), and I managed to pluck out two pairs that were close~ish.

I unfortunately have no photos from this testing session, but it sounded… off. Some quick measurements and I was able to get the tubes balanced before running out the potentiometer. Still, I was at the extreme for the bias adjustment and I knew that (a) I wanted to run old-stock tubes without paying $hundreds for them, and (b) new 7189 tubes are expensive!

I put on my robe and wizard hat.

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